Rare earth elements are recently attracting more and more interests due to their dramatic effect as alloying elements. However the study regarding the phase equilibria between rare earth elements are still rare. The thermodynamically stable phases within the similar to 50 at.%Y region have been investigated for the La-Y binary system using differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy in the present work. The samarium-type delta-LaY has been found to be unstable while a pure Y phase with fcc crystal structure has been observed. A new invariant reaction has been determined to be bcc-(La,Y) + fcc-Y -> dhcp-(La,Y) at 717 +/- 3 degrees C.